Solid particle erosion of the components of a steam turbine occurs due to carryover of particles from the steam boiler and piping upstream of the turbine. The solid particles become entrained in the steam flow path. As they pass through the steam turbine, the particles cause damage to both the stationary and rotating parts of the turbine which degrades steam turbine performance and mechanical reliability. The solid particles may be deposited throughout the steam path or may exit the steam path into steam extractions that feed the feed water heaters of the cycle. However, since the particles are transported by the main steam flow through the steam turbine steam path, they have the opportunity to inflict considerable damage along the steam path before they are deposited or expelled from the main steam flow. This damage can include erosion of the rotating and stationary buckets and partitions respectively, erosion of the rotating tip covers or tenons, erosion of tip sealing devices such as spill strips and erosion of stationary structures over the tips of the rotating buckets.
Referring to FIG. 1, typical steam turbine stages of a conventional steam turbine are illustrated and generally designated 10. Two stages of the steam turbine are illustrated, for example, a first stage generally designated 12 and a second stage generally designated 14. The first stage 12 includes a diaphragm 16 having an inner web 18, an outer ring 20, and a plurality of circumferentially spaced stator vanes or partitions 22 therebetween. The first stage also includes buckets 24 secured to a rotor 26. The tips of the buckets 24 rotate past sealing devices 28 formed on an axially extending appendage 30 of the outer ring 20. The inner web 18 of the first stage diaphragm includes sealing segments 32, in this instance, mounting labyrinth seal teeth 34 for sealing about the rotor 26. The second stage 14 is similar and includes a diaphragm 36, an inner web 38, an outer ring 40, partitions 42 circumferentially spaced one from the other and disposed between the inner web and outer ring, the outer ring 40 having an appendage 44 overlying tips of buckets 46 mounted on the rotor 26. It will be appreciated that the steam flows through the illustrated stages in the direction of the arrow 48 rotating the rotor 26, thereby enabling useful work to be derived from the steam turbine.
Various apparatus and methods have been proposed and utilized to minimize the impact of the solid particles on the rotating and stationary parts of steam turbines. For example, in U.S. Pat. No. 4,776,765 a protective device is disposed over a portion of the suction side of the partition to prevent solid particle erosion of the trailing edge of the partition due to rebound of particles from the leading edge of the buckets. Other apparatus and methods for minimizing or eliminating solid particle erosion in steam turbines include solid particle erosion resistant coatings such as disclosed in U.S. Pat. Nos. 4,704,336 and 4,615,734.
An additional conventional apparatus is shown in FIGS. 2 and 3, wherein like reference numerals are applied to like parts as in the conventional steam turbine construction illustrated in FIG. 1. This apparatus provides for the removal of a portion of the solid particles from the main steam flow so as to minimize damage to downstream steam path components. Generally, holes and passageways are provided in the component parts to divert a portion of the steam and hence the solid particles carried by the steam about the rotating parts. Particularly, one or more holes 60 are provided in the appendage 30 for diverting a portion of the steam flowing through the steam path through the hole 60. The hole 60 includes an inlet opening 64 upstream of the buckets 24 of the first stage and a second portion 66 on an opposite sides of a sealing device 68. A passage 70 extends through the sealing segment 68 in communication with the hole portions 64, 66, thereby constituting a through-passageway in appendage 30 for bypassing steam about the rotating parts. As illustrated, the hole portion 66 exits into a passageway 72 extending through the outer ring 40 of the second stage and on to a steam extraction passage 73.
One drawback associated with this arrangement is that the particles do not easily enter the inlet opening 64. In other words, the shape of the diaphragm surface adjacent the inlet opening does not effectively direct particles to the inlet opening. Thus, particles forced near the inlet opening by centrifugal action are often still deposited under the covers of the rotating buckets, which degrades mechanical integrity of the rotating buckets.
Further, even when particles successfully enter the inlet opening 64, they are not easily passed through the hole 60. Centrifugal action causes the particles to move radially outwardly toward the inlet opening 64; however, the hole 60 is positioned perpendicularly to the inlet opening 64 thereby requiring the particles to make a sharp turn into the hole 60. As such, passage of particles through the hole 60 is hindered by this configuration.
While many of these and other efforts to minimize or eliminate solid particle erosion have been tried in the past, solid particle erosion in steam turbines remains a continuing problem for the various parts along the steam path. Accordingly there is a need for a device to effectively minimize solid particle erosion of steam turbine components.